(Reuters) - A British-Iranian charity worker serving a jail sentence in Tehran received a letter from ex-prime minister David Cameron that showed she had ties to the British government, a prosecutor said on Tuesday, according to Mizan, the news site of the Iranian judiciary.

Nazanin Zaghari-Ratcliffe, a project manager with the Thomson Reuters Foundation, a charity organisation, is serving a five-year jail sentence after being convicted of plotting to overthrow Iran's clerical establishment.

The prosecutor, Abbas Jafari Dolatabadi, said the letter demonstrated Zaghari-Ratcliffe's importance to the British authorities but he did not say when it was sent or provide any other details about it.

A spokeswoman at the British Foreign Office said she was not immediately able to comment on whether Cameron had written a letter, but confirmed that both he and his successor, Theresa May, had raised the Zaghari-Ratcliffe case with Iranian authorities.

"We will continue to raise all our dual national detainees, including Mrs Zaghari-Ratcliffe's case, with the Iranian government at every available opportunity," she added.

Zaghari-Ratcliffe was arrested by the elite Revolutionary Guards in April 2016 at a Tehran airport, as she was about to return to Britain with her two-year-old daughter after a family visit.

Her family and the Thomson Reuters Foundation have both denied the charges against her. Thomson Reuters is a charity organization that is independent of Thomson Reuters and operates independently of Reuters News.

Last week her family said Iranian authorities had opened a new case against Zaghari-Ratcliffe, leveling charges that could carry a sentence of 16 additional years in prison. The new charges include joining and receiving money from organizations working to overthrow the Islamic Republic, and attending a demonstration outside the Iranian Embassy in London, the family said.

The Foreign Office spokeswoman referred Reuters to a statement it issued last week in which it "expressed concern" that Zaghari-Ratcliffe was facing additional charges and said it was seeking more information from the Iranian authorities.

On Tuesday Jafari Dolatabadi also said Zaghari-Ratcliffe was responsible for teaching online journalism for the BBC Persian language service "with the goal of attracting and teaching individuals for propaganda operations against Iran", Mizan reported.

Francesca Unsworth, director of the BBC World Service Group, said earlier this year that Zaghari-Ratcliffe had never worked for the BBC's Persian service.